IGBTs include a bipolar transistor and a MOSFET. The bipolar emitter is on the bottom of the device (although in various descriptions, the bottom terminal is sometimes referred to as a “collector” where the IGBT high-voltage terminal (the IGBT collector) is connected to the region which functions as the emitter of the integral bipolar transistor), and operates to inject minority carriers into the bipolar base, thereby filling this region with a plasma of holes and electrons to facilitate high current density. Bipolar conduction in IGBTs provides an advantage in terms of current per unit area, but results in a disadvantage in terms of switching speed. The excess carriers in the hole-electron plasma do not instantaneously disappear after the current has stopped flowing, and the device cannot return to the off-state and support a high voltage with low leakage until the excess carriers are gone. Therefore, if the device is to be designed for fast switching, it is necessary to build in a mechanism to provide rapid removal of excess carriers.
Current is carried by both holes and electrons in bipolar devices having an emitter region of one conductivity type adjacent a base region of the opposite conductivity type. During conduction, the emitter injects its majority carriers as minority carriers into the base region. Entry of these minority carriers into the base permits the entrance of equal quantities of base majority carriers, and thus the total carrier concentration in the base region can rapidly exceed the base dopant concentration. The result is conductivity modulation of the base region, in which the base conductivity becomes much higher, and resistivity much lower, than the background value. This conductivity-modulated bipolar conduction advantageously permits the device to carry a much higher current density than a similar unipolar device. In an IGBT, the emitter operates to emit carriers into a voltage supporting region at the bipolar base, and fast switching IGBTs can be built using emitter shorting contacts connecting the emitter to the base for excess carrier removal to turn the device off quickly. In general, a resistor or low impedance contact can be provided between the emitter and the base, in parallel with the emitter-base junction. This emitter-base shunt resistor may be connected externally, or may be built into the structure.
The excess carriers can thus be removed quickly from base region to interrupt the current flow for fast switching applications. One way to do this is to create recombination centers to provide mid-band energy levels where holes and electrons can recombine. Recombination centers can be provided by doping the crystal with heavy metals, such as gold or platinum, or by bombarding the crystal with high energy neutrons, protons, electrons, or gamma rays to produce localized damage sites. Shorted emitters have several advantages over recombination centers. Recombination centers are more effective at removing carriers at high carrier densities than at low densities, while emitter shorts are more effective at low carrier densities, which is the condition during switching. As carrier density increases, more carriers encounter recombination centers and recombine, but this limits the level of conductivity modulation, and thereby increases the on-voltage. Emitter shorts have better impact at low carrier densities. When current is low enough that the voltage drop on the emitter-shorting resistor is less than the 0.6-0.8 volt built-in offset voltage of the junction, almost all majority carriers flow through the shorting contact or resistor rather than crossing the junction and injecting minority carriers. With only recombination centers, majority carriers continue crossing the junction and injecting minority carriers even down to very low current levels, thereby slowing the device turn-off. With the emitter shorts, minority carrier injection ceases as soon as the drop across the shorting resistor falls below the 0.6-0.8 volt level. Emitter shorts thus reduce the low-current gain, but have only a small effect on high current gain.
High voltage IGBT devices are used to switch high voltage electrical power, and certain applications require fast switching times for both turn-on and turn-off. For a given switching speed, an IGBT made with emitter shorts can have a lower on-voltage at both low-current and high-current levels than an IGBT made with only recombination centers. However, high voltage devices with high switching speeds require control over the drift region thickness, and conventional techniques provide no way for backside processing to create emitter shorts for devices with fairly thin drift regions necessary to achieve high switching speeds.